Like most known tyrannosaurids, Gorgosaurus was a bipedalpredator weighing more than two metric tons as an adult; dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was most closely related to Albertosaurus, and more distantly related to the larger Tyrannosaurus. Gorgosaurus and Albertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts consider G. libratus to be a species of Albertosaurus; this would make Gorgosaurus a junior synonym of that genus.

Gorgosaurus lived in a lush floodplain environment along the edge of an inland sea. It was an apex predator, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Although these animals were roughly the same size, there is some evidence of niche differentiation between the two. Gorgosaurus is the best-represented tyrannosaurid in the fossil record, known from dozens of specimens. These plentiful remains have allowed scientists to investigate its ontogeny, life history and other aspects of its biology.

Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, closer in size to Albertosaurus and Daspletosaurus. Adults reached 8 to 9 m (26 to 30 ft) from snout to tail.[1][2] Paleontologists have estimated full-grown adults to weigh more than 2.4 tonnes (2.6 short tons),[3] perhaps approaching 2.8 tonnes (3.1 short tons).[4] The largest known skull measures 99 cm (39 in) long, just slightly smaller than that of Daspletosaurus.[1] As in other tyrannosaurids, the skull was large compared to its body size, although chambers within the skull bones and large openings (fenestrae) between bones reduced its weight. Albertosaurus and Gorgosaurus share proportionally longer and lower skulls than Daspletosaurus and other tyrannosaurids. The end of the snout was blunt, and the nasal and parietal bones were fused along the midline of the skull, as in all other members of the family. The eye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera. A tall crest rose from the lacrimal bone in front of each eye, similar to Albertosaurus and Daspletosaurus.[2] Differences in the shape of bones surrounding the brain set Gorgosaurus apart from Albertosaurus.[5]

Gorgosaurus teeth were typical of all known tyrannosaurids. The eight premaxillary teeth at the front of the snout were smaller than the rest, closely packed and D-shaped in cross section. In Gorgosaurus, the first tooth in the maxilla was also shaped like the premaxillary teeth. The rest of the teeth were oval in cross section, rather than blade-like as in most other theropods.[2] Along with the eight premaxillary teeth, Gorgosaurus had 26 to 30 maxillary teeth and 30 to 34 teeth in the dentary bones of the lower jaw. This number of teeth is similar to Albertosaurus and Daspletosaurus but is fewer than those of Tarbosaurus or Tyrannosaurus.[6]

Life restoration

Gorgosaurus shared its general body plan with all other tyrannosaurids. Its massive head was perched on the end of an S-shaped neck. In contrast to its large head, its forelimbs were very small. The forelimbs had only two digits, although a third metacarpal is known in some specimens, the vestigial remains of the third digit seen in other theropods. Gorgosaurus had four digits on each hindlimb, including a small first toe (hallux) which did not contact the ground. Tyrannosaurid hindlimbs were long relative to overall body size compared with other theropods.[2] The largest known Gorgosaurusfemur measured 105 cm (41 in) long. In several smaller specimens of Gorgosaurus, the tibia was longer than the femur, a proportion typical of fast-running animals.[1] The two bones were of equal length in the largest specimens.[7] The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips.[2]

In 2001, paleontologist Phil Currie reported skin impressions from the holotype specimen of G. libratus. He originally reported the skin as being smooth and lacking the scales found in other dinosaurs; similar to the secondarily featherless skin found in large modern birds.[8] However, this original interpretation was found to be an exaggeration based on the relatively fine scalation of the Gorgosaurus skin impression (approximately as fine as a Gila monster's).[9] In the Encyclopedia of DinosaursKenneth Carpenter pointed out that traces of skin impressions from the tail of Gorgosaurus showed similar small rounded or hexagonal scales.[10]

Gorgosaurus is classified in the theropod subfamily Albertosaurinae within the family Tyrannosauridae. It is most closely related to the slightly younger Albertosaurus.[6] These are the only two definite albertosaurine genera that have been described, although other undescribed species may exist.[5]Appalachiosaurus was described as a basaltyrannosauroid just outside Tyrannosauridae,[12] although American paleontologist Thomas Holtz published a phylogenetic analysis in 2004 which indicated it was an albertosaurine.[2] More recent, unpublished work by Holtz agrees with the original assessment.[13] All other tyrannosaurid genera, including Daspletosaurus, Tarbosaurus and Tyrannosaurus, are classified in the subfamily Tyrannosaurinae. Compared to the tyrannosaurines, albertosaurines had slender builds, with proportionately smaller, lower skulls and longer bones of the lower leg (tibia) and feet (metatarsals and phalanges).[6][14]

The close similarities between Gorgosaurus libratus and Albertosaurus sarcophagus have led many experts to combine them into one genus over the years. Albertosaurus was named first, so by convention it is given priority over the name Gorgosaurus, which is sometimes considered its junior synonym. William Diller Matthew and Barnum Brown doubted the distinction of the two genera as early as 1922.[15]Gorgosaurus libratus was formally reassigned to Albertosaurus (as Albertosaurus libratus) by Dale Russell in 1970,[1] and many subsequent authors followed his lead.[12][16] Combining the two greatly expands the geographical and chronological range of the genus Albertosaurus. Other experts maintain the two genera as separate.[2] Canadian paleontologist Phil Currie claims there are as many anatomical differences between Albertosaurus and Gorgosaurus as there are between Daspletosaurus and Tyrannosaurus, which are almost always kept separate. He also notes that undescribed tyrannosaurids discovered in Alaska, New Mexico and elsewhere in North America may help clarify the situation.[5]Gregory S. Paul has suggested that Gorgosaurus libratus is ancestral to Albertosaurus sarcophagus.[17]

Matthew and Brown also described a fifth skeleton (AMNH 5664), which Charles H. Sternberg had collected in 1917 and sold to their museum. It was smaller than other Gorgosaurus specimens, with a lower, lighter skull and more elongate limb proportions. Many sutures between bones were unfused in this specimen as well. Matthew and Brown noted that these features were characteristic of juvenile tyrannosaurids, but still described it as the holotype of a new species, G. sternbergi.[7] Today's paleontologists regard this specimen as a juvenile G. libratus.[2][5] Dozens of other specimens have been excavated from the Dinosaur Park Formation and are housed in museums across the United States and Canada.[1][5]G. libratus is the best-represented tyrannosaurid in the fossil record, known from a virtually complete growth series.[2][22]

In 1856, Joseph Leidy described two tyrannosaurid premaxillary teeth from Montana. Although there was no indication of what the animal looked like, the teeth were large and robust, and Leidy gave them the name Deinodon.[23] Matthew and Brown commented in 1922 that these teeth were indistinguishable from those of Gorgosaurus, but in the absence of skeletal remains of Deinodon, opted not to unequivocally synonymize the two genera, provisionally naming a ?Deinodon libratus.[15] Although Deinodon teeth are very similar to those of Gorgosaurus, tyrannosaurid teeth are extremely uniform, so it cannot be said for certain which animal they belonged to. Deinodon is usually regarded as a nomen dubium today.[22] Additional likely synonyms of G. libratus and/or D. horridus include Laelaps falculus, Laelaps hazenianus, Laelaps incrassatus, and Dryptosaurus kenabekides.[24]

In the middle stages of the Dinosaur Park Formation, Gorgosaurus lived alongside a rarer species of the tyrannosaurine Daspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similar-sized predators in modern predator guilds are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well understood.[32] In 1970, Dale Russell hypothesized the more common Gorgosaurus actively hunted fleet-footed hadrosaurs, while the rarer and more troublesome ceratopsians and ankylosaurians (horned and heavily armoured dinosaurs) were left to the more heavy built Daspletosaurus.[1] However, a specimen of Daspletosaurus (OTM 200) from the contemporaneous Two Medicine Formation of Montana preserves the digested remains of a juvenile hadrosaur in its gut region,[33] and another bonebed contains the remains of three Daspletosaurus along with the remains of at least five hadrosaurs.[34]

Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other.[32] However, Gorgosaurus appears more common in northern formations like Dinosaur Park, with species of Daspletosaurus more abundant to the south. The same pattern is seen in other groups of dinosaurs. Chasmosaurine ceratopsians and hadrosaurine hadrosaurs are also more common in the Two Medicine Formation of Montana and in southwestern North America during the Campanian, while centrosaurine and lambeosaurines dominate in northern latitudes. Holtz has suggested this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and hadrosaurines. At the end of the later Maastrichtian stage, tyrannosaurines like Tyrannosaurus rex, hadrosaurines like Edmontosaurus and chasmosaurines like Triceratops were widespread throughout western North America, while lambeosaurines were rare and albertosaurines and centrosaurines had gone extinct.[2]

A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids. Gorgosaurus is shown in blue. Based on Erickson et al. 2004.

Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids using bone histology, which can determine the age of a specimen when it died. A growth curve can be developed when the ages of various individuals are plotted against their sizes on a graph. Tyrannosaurids grew throughout their lives, but underwent tremendous growth spurts for about four years, after an extended juvenile phase. Sexual maturity may have ended this rapid growth phase, after which growth slowed down considerably in adult animals. Examining five Gorgosaurus specimens of various sizes, Erickson calculated a maximum growth rate of about 50 kg (110 lb) per year during the rapid growth phase, slower than in tyrannosaurines like Daspletosaurus and Tyrannosaurus, but comparable to Albertosaurus.[35]

Gorgosaurus spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years.[35] This, along with the complete lack of predators intermediate in size between huge adult tyrannosaurids and other small theropods, suggests these niches may have been filled by juvenile tyrannosaurids. This pattern is seen in modern Komodo dragons, whose hatchlings start off as tree-dwelling insectivores and slowly mature into massive apex predators capable of taking down large vertebrates.[2] Other tyrannosaurids, including Albertosaurus, have been found in aggregations that some have suggested to represent mixed-age packs, but there is no evidence of gregarious behavior in Gorgosaurus.[34][36]

Bob Bakker and a skeleton with several bone injuries, from the "Dinosaur Mummy: CSI" exhibit at the HMNS

Several pathologies have been documented in the Gorgosaurus libratusholotype, NMC 2120. These include the third right dorsal rib, as well as healed fractures on the 13th and 14th gastralia and left fibula. Its fourth left metatarsal bore roughened exostoses both in the middle and at the far end. The third phalanx of the third right toe is deformed, as the claw on that digit has been described as "quite small and amorphous." The three pathologies may have been received in a single encounter with another dinosaur.[37]

Another specimen cataloged as TMP94.12.602 bears multiple pathologies. A 10 cm (3.9 in) longitudinal fracture is present in the middle of the right fibula's shaft. Multiple ribs bear healed fractures and the specimen had a pseudoarthorticgastralium. Lesions from a bite received to the face were present and showed evidence that the wounds were healing before the animal died.[37]

TMP91.36.500 is another Gorgosaurus with preserved face bite injuries but also has a thoroughly healed fracture in the right fibula. Also present was a healed fracture in the dentary and what the authors describing the specimen referred to as "a mushroom-like hyperostosis of a right pedal phalanx." Ralph Molnar has speculated that this may be the same kind of pathology afflicting an unidentified ornithomimid discovered with a similar mushroom shaped growth on a toe bone.[37]

Another specimen has a poorly healed fracture of the right fibula, which left a large callus on the bone. In a 2001 study conducted by Bruce Rothschild and other paleontologists, 54 foot bones referred to Gorgosaurus were examined for signs of stress fracture, but none were found.[37][38]

^ abLambe, Lawrence M. (1914). "On the fore-limb of a carnivorous dinosaur from the Belly River Formation of Alberta, and a new genus of Ceratopsia from the same horizon, with remarks on the integument of some Cretaceous herbivorous dinosaurs". Ottawa Naturalist27: 129–135.

^ abLambe, Lawrence M. (1914). "On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of Stephanosaurus marginatus from the same horizon". Ottawa Naturalist28: 13–20.

^Leidy, Joseph. (1856). "Notice of remains of extinct reptiles and fishes, discovered by Dr. F.V. Hayden in the badlands of the Judith River". Proceedings of the Academy of Sciences in Philadelphia8: 72–73.